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### Course: Physics library>Unit 1

Lesson 5: Old videos on projectile motion

# Projectile motion (part 3)

An example of solving for the final velocity when you know the change in distance, time, initial velocity, and acceleration. Created by Sal Khan.

## Want to join the conversation?

• What is the difference between constant acceleration and constant velocity?
• constant acceleration means the velocity is changing at a constant rate it can increase or decrease but constant velocity means that if a particle is having constant velocity it will move with that velocity only until and unless any external force is applied to it
• Are we supposed to memorize each and every formula for time? I need to know how to get time when given different values ( distance, initial velocity, final velocity, displacement, ect.) Can you give me a list of formulas to use in each scenario? Thanks!!!
• it's quite simple to remember.
1. s=ut+1/2at^2
2. v^2=u^2+2as
3. v=u+at
4.t=v-u/a
where,v is final velocity,u initial velocity,t the time,a acceleration and s the distance.
• Hi Sal,
You have taken upwards positive and downwards negative. Could we take upwards negative and downwards positive?
• Yes, since coordinates have no effect on physics.
• At Salman Khan told that Avg. Velocity = Change in distance by change in time, But I thought that it was Initial vleocity+ Final Velocity /2? Which one is correct?
• Your formula only works when acceleration is constant.
• Isn't sal supposed to use displacement (delta S) instead of distance
(1 vote)
• In he stated that the formula for average velocity is (Vf+Vi)/2 assuming the acceleration is constant , but what if the acceleration wasnt constant and we were dealing with a jerky motion what the formula for average velocity might be ?
(1 vote)
• Then you would need to calculate the average in a different way, perhaps with calculus.
(1 vote)
• My problem has angles and no velocity is given. How do I figure it out when I'm given an angle and how far down the projectile went from the horizon?
(1 vote)
• i am unable to do the problems in our text using the formula Sal says?what should i do?
(1 vote)
• When we say "change in displacement 'delta S' ", do we mean the change in the displacement form a particular reference point?